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Stringy fields may make the universe swell faster

By Jacob Aron

Not quite one universe in a million

(Image: Superstock)

DARK energy, the mysterious force thought to be responsible for the fact the universe’s expansion is accelerating, might come from a series of exotic fields. This notion, which has its origins in string theory, could explain why it was only after galaxies formed that the rate of expansion began to increase.

Dark energy could simply be a property of space-time, called the cosmological constant, which appears as a term added into the equations of general relativity. But, the trouble is that to make the equations balance, the constant should be 120 orders of magnitude larger than observations of the universe suggested it actually is.

This gigantic discrepancy is a major headache for cosmologists, so some have turned to more complex explanations. One invokes a field called quintessence that supposedly fills the universe and provides the energy necessary for expansion. Crucially, this field changes strength over time, which would explain why cosmic expansion hasn’t always been accelerating.

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But it leaves another problem&colon; why did acceleration switch on precisely when it did? One explanation is that if it hadn’t, we wouldn’t be here to see it. But counting up the other equally probable options makes our universe just one of 10120 possibilities, a fine-tuning that physicists find equally unsatisfactory.

Now Marc Kamionkowski of Johns Hopkins University in Baltimore, Maryland, and his colleagues say they have an explanation that solves both problems, thanks to something called the string axiverse. String theory, an overarching term for theories with more than three dimensions, suggests there could be hundreds of so-called axion fields that permeate the universe. In the early universe these fields wouldn’t have an effect, but one by one, they would turn on, giving rise to particles of different masses.

Kamionkowski says each field has a chance of becoming quintessence and accelerating cosmic growth, but the chance any particular field has decreases with the total number of fields. “All the fields exist at all times, but in the beginning they are frozen, just sitting on the bench.” It is a bit like rolling a series of dice – if you roll a six, it is unlikely to be the first six to occur if many other dice have been rolled already.

The team calculated that it would have to be the 44th axion field to turn on that became quintessence. The odds of this happening are 1 in 500, they say (arxiv.org/abs/1409.0549). In other words, our universe was still unlikely, but much less so than in a scenario without axions.

The other fields could become ordinary and dark matter, thus explaining everything in the universe, says Kamionkowski – although the team is still working out the details. “We’re going to keep busy with this for a little while.” Telescopes looking for changes in the energy density of space could provide hints that dark energy is indeed from the string axiverse, he adds.

John March-Russell of the University of Oxford, who helped conceive the string axiverse, agrees it could explain dark energy, but doesn’t quite solve the fine-tuning problem. Kamionkowski admits the model rests on assumptions that are just one of many possible choices, but thinks that the arguments for different assumptions are mostly philosophical. “It’s turtles all the way down!”

This article appeared in print under the headline “String fields speed cosmic growth”